Nutrient Enrichment and Eutrophication Potential of Agricultural Runoff in Otuoke, Bayelsa State, Nigeria

Abstract

The present study evaluated the extent of nutrient enrichment and eutrophication potential of the Otuoke River in Bayelsa State as influenced by agricultural runoff. Nine surface water samples were collected from three monitoring stations located upstream, midstream, and downstream during the 2025 wet season and analysed according to the APHA (1995) standard method. Selected physicochemical parameters—pH, dissolved oxygen (DO), biological oxygen demand (BOD₅), nitrate, phosphate, ammonium, and chlorophyll-a—were determined using a Hanna HI98194 multi-parameter probe and Shimadzu UV-1800 spectrophotometer. Results showed a decrease in DO, which dropped from 6.8 mg/L at the upstream site to 4.1 mg/L at the downstream site, with a concurrent increase in BOD₅, rising from 2.8 to 5.4 mg/L, indicating organic enrichment occurred. Nutrient concentrations showed downstream increases in nitrate and phosphate, with nitrate ranging, and ammonium, indicators of runoff and fertiliser, increasing from 0.31 to 0.68 mg/L. Chlorophyll-a concentration ranged from 8.4 to 23.5 µg/L, while the Secchi depth decreased from 1.8 to 0.7 m. The Trophic State Index (TSI) recorded values ranging from 45.2 to 67.5, classifying the river from mesotrophic to eutrophic. The study revealed that strong positive correlations between nutrient concentration and chlorophyll-a (r ≥ 0.88) indicate nutrient loading was the primary factor controlling algal productivity and consequently reducing oxygen levels in the Otuoke River. The study concluded that agricultural practices are significant contributors to the trophic status of the river.